1. Field of the Invention
The present invention relates to a composition and method for fabricating composite resins with reduced and zero volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions. More specifically, the present invention relates to composites or resins incorporating fatty acid monomers for use as a repair resin.
2. Brief Description of the Prior Art
Composite resins used to repair equipment, vehicles, planes and watercraft typically contain 10%-20% by weight hazardous air pollutants (HAP) and volatile organic compounds (VOC). HAP and VOC chemicals are used to reduce repair resin viscosity to form a resin that is both easy to blend and easy to apply. Generally, composite resins are composed of a polymeric binder component and a hardener. The polymeric binder generally includes a cross-linking agent, such as vinyl ester or unsaturated polyester, a reactive diluent, such as styrene, a free-radical decomposition promoter, a free-radical inhibitor, and various inorganic additives, such as talc, magnesium carbonate, chopped glass fiber and cabosil. Not including the additives, the HAP and VOC content constitutes about 20%-50% of the resin binder content by weight. The polymeric binder is mixed with the hardener which typically comprises a free-radical initiator and surfactants.
In an effort to minimize the environmental damage caused by HAP emissions, the Environmental Protection Agency has established new regulations limiting the HAP content of composite materials. The regulations require compliance with facility wide emissions limits by 2008 and continuous emissions monitoring of all HAP-containing composite materials. Therefore there exists a need to develop composite resins capable of mitigating HAP emissions and meeting the proposed guidelines.
This legislation particularly affects the manufacturing of composite resins used for repairing military equipment. A recent 2006 report to the Army entitled, “Miscellaneous Adhesives and Sealants Technology Thrust Area”, states that currently, there exist no environmentally friendly repair resins. Consequently, the military generates thousands of pounds of hazardous waste from expired resins annually. HAP emissions from vehicle repairs are one of the largest sources of emissions from miscellaneous coatings in the Army. Moreover, because it is impractical to use enclosures and trapping devices, field repair resins are typically cured in the open, generating a significant amount of HAP emissions.
Additionally, there exists a need to develop a more durable composite repair resin. The high reactivity of the current repair resin chemistry results in a short shelf-life of less than 1 year; thermal and mechanical properties decrease rapidly upon expiration of the shelf-life. This is particularly problematic in view of the fact that military equipment must be able to withstand extreme conditions. Therefore typical military repair resins, such as Bondo™, 6294 Dent Filler, 887 Metal-2-Metal Reinforcement and other vinyl ester composite resins, lack sufficient durability and material properties to endure military applications.
Military equipment is frequently subject to damaged by impact of a foreign object, ballistic impact, moisture intrusion and expansion, corrosion, collision and maintenance-induced damage. The degree of damage may range from: light damage, requiring only aesthetic repairs and coating repairs; moderate damage, requiring repair of delaminations, small patches and edge repairs, to heavy damage, requiring full depth, core and substructure repairs.
For a typical light to moderate field repair, any remaining coating in the repair area is removed by hand sanding or portable tools. The damaged part is then cut out in an appropriate configuration, often circular. Scarfing, removal of top layers of material done at a shallow angle, is commonly done by hand. The surface is then sanded further and cleaned using an available solvent. Composite repair resins, such as some Bondo™ products, containing short reinforcing fibers can then be applied to the damage zone. The resin cures at room temperature. Light repairs can be done in a similar manner using Bondo™ products and similar composite repair resins without much, if any, scarfing.
Depot repair is typically a bit more elaborate. Rather than using simple repair resins for moderate or heavy damage, the damage zone will be filled with fibers or honeycomb and vacuum infused or cured using wet lay-up. This allows the use of more elaborate resin systems, such as phenolics, and autoclave cure. However, for most light repair and some moderate repair, Bondo™ and similar resins are used for the repair. Not only are these repair resins used in composite structures, they are also used to do light and moderate repair to non-armor metal structures, such as body paneling, by filling in holes and dents in a manner similar to that used for composite structures.
In light of the need for durable reduced HAP repair resins for military as well as commercial use, various environmentally friendly material alternatives such as electron-beam (E-beam) curable repair resins have been proposed. Unfortunately, E-beam instruments impose a high capital cost and are not portable, making military field repair impractical. Furthermore, E-beam curable resins are not a drop-in alternative to current repair resins, and are therefore not highly desired by the end users.
Epoxy resins were also found inadequate, in part because epoxy resins cost approximately four times the amount of vinyl ester (VE) and unsaturated polyester (UPE) resins. In addition, epoxy resins are usually formulated in two part mixtures that need to be added in precise ratios. Inaccurate ratios or poor mixing can result in poor and inconsistent properties. Epoxies can be cured using agents, such as a dicyandiamide, where exact mix ratios are not important, but cure of these systems does not occur until elevated temperatures, which is impractical for field repair.
Researches have also contemplated composite repair resins that reduce HAP and VOC components such as styrene. Simply reducing the styrene content in composite resins, however, causes one of two significant problems. If the resin is filled to the same extent with inorganic fillers for the low styrene formulation as it is with the regular formulation, the low HAP repair resin will have an unacceptably high viscosity. Alternatively, lower filler contents can be used, but these will detrimentally affect the stiffness of the resulting repair.
It has also been proposed to replace the commonly used HAP component styrene with other components, such as 2-hydroxymethacrylate. These substitutions generally produce inferior resin viscosity and properties in comparison to styrene-based thermosetting resins. Moreover, 2-hydroxymethacrylate produces significant VOC emissions. Ortho and para-methyl styrene have lower volatilities than styrene; however, these chemicals still produce significant VOCs and would probably be classified as HAPs if used on a large scale.
U.S. Pat. Nos. 4,918,120 and 5,286,554 disclose incorporating additives, such as paraffin waxes, to suppress styrene emissions. These resins, however, suffer from poor polymer performance and poor interfacial adhesion in fiber-matrix composites. Furthermore, studies have shown that these additives do not effectively decrease styrene emissions during the time-scale of use.
In an effort to develop a practical environmentally friendly composite resin, fatty acid monomers (FAM) have also been considered. U.S. patent publication no. 2005/0277734 discloses a polyurethane sealant including a fatty acid ester. Additionally, U.S. patent publication no. 2005/0250923 also discloses a technology for compatibilizing fatty acid monomers and vinyl ester and unsaturated polyester resins. The composite resin attaches free-radically polymerizable functionality to fatty acids, while adding other functionality to make the fatty acid monomers soluble in VE and UPE resins. Composite resin formulations incorporating fatty acid monomers reduced styrene content up to about 78%. Although these resins reduce HAP and VOC emissions, the patents do not disclose the capability of completely eliminating HAP and VOC components or emissions. Nor do these patents present inorganically filled formulations that are useable for repair applications.
There are a number of reasons why the study and development of fatty acid-based monomers for use in composite resins is important. First, fatty acid monomers can be used to replace some or all of the styrene used in liquid thermosetting resins. Fatty acid monomers are excellent alternatives to styrene because of their low cost and low volatility. Furthermore, fatty acids are derived from plant oils, and are therefore a renewable resource. Thus, not only does the use of fatty acid monomers in liquid molding resins reduce health and environmental risks, but it also promotes global sustainability.
Fatty acids and triglycerides have been used in a number of polymeric applications. The preparation of epoxidized and hydroxylated fatty acids has been reviewed by many researchers, including Gunstone, Litchfield, Swern, etc. Epoxidized and acrylated triglycerides have been used as plasticizers and toughening agents. In fact, the largest non-food use of triglycerides is the use of epoxidized soybean and linseed oils as plasticizers in poly(vinyl chloride). Epoxidized triglycerides have also been studied for use as toughening agents in epoxy polymers.
The production of free radically reactive plant oil-based monomers is a more recent invention. Nevin patented the preparation of acrylated triglycerides in U.S. Pat. No. 3,125,592, which can be homopolymerized or copolymerized with other free-radically reactive monomers. These acrylated triglycerides have been used in coatings, inks, toughening agents, and adhesives. Using this technology, adhesives have been made from fatty acid methyl esters. In addition, thermosetting liquid molding resins have been made using chemically modified plant oils as cross-linking agents in thermosetting resins (U.S. Pat. No. 6,121,398). Anhydrides, such as phthalic anhydride, have been used to form air curable coatings (Japanese Patent nos. 73-125724, 74-103144, 80-62752, and 81-64464). In addition, the use of maleic anhydride for making free-radically reactive triglycerides has been patented (U.S. Pat. No. 6,121,398). However, until now, fatty acids have not been used as reactive diluents in thermosetting liquid molding resins.